Researchers in holography have long been interested in ways to produce holograms from incoherent data, such as from a set of conventional transparancies of many views of an object or transparancies representing a stack of parallel cross sections of the object. This interest has intensified with the increasing importance of three (or more) dimensional data in electronic or computer based systems, data which may correspond to real images, as from a 3-D medical imaging device, or data which may be abstract output from computer simulations, solutions of differential equations and the like. The prior art has described various synthetic hologram methods to produce holograms of electronic data; these methods often required the steps of producing a particular perspective view or sectional representation of the 3-D data onto a CRT screen, preparing a transparency of each such image component, and producing a hologram according to some predefined synthetic hologram protocol. More recently an "incoherent to coherent converter", such as a liquid crystal light valve or magnetooptical modulator has in some cases come to replace the more time consuming CRT- transparency-subsequent coherent illumination method to obtain coherent components for the required multiplex holograms.
There are numerous references which describe synthetic hologram protocols, for instance the Handbook of Optical Holography, or any number of patents in the subclass "Synthetic Holography". These prior art methods all require either a significant number of separate exposures--that is, a large number of separate holograms must be made, one from each of perhaps a thousand perspective views for Cross type multiplex holograms, or one separate hologram of each section of an object for synthetic sectional holograms. Each of these methods requires that multiple exposures be made of each component, a fact that places an unnecessary burden upon the electronic processing system for Cross type holograms. In other techniques for multiple perspective based synthetic holography, one may synthesize a coherent three dimensional image, as in lens sheet arrays, but in order to synthesize a 3-D coherent wavefront one requires an enormous number of 2-D coherent inputs--one for each lens of the array--a process that would require thousands of simultaneous inputs, each uniquely calculated electronically, an unfeasible proposition. Part of the problem with these approaches is that each input component to both the cross holograms or lens sheet arrays contains significant redundant information--a given radiating point of the synthesized image is contained in numerous individual components to create an image of that point from many different perspectives.
Sectional based synthesis offers several unique advantages over the above. For one, there is no redundancy of information in each section; each section contains only information related to a particular distance from the observer so that any given point in a 3-D volume is in general represented by only one point in one section. Another advantage is that the human visual system will interpolate between sections to give an impression of a 3-D image continuous in the depth dimension, even when this 3-D image is composed of as few as 16 separate planes disposed at discrete distances along the depth (or z) axis. Thus, for example, even 3-D data represented electronically as a set of (x,y,z) coordinates with a significant number of different z values may be compressed, for display purposes, by projecting a given point to one of the discrete planar sections: the resulting 3-D image will still be perceived as continuous.
The most serious limitation to synthetic sectional holography is that a separate hologram must be made of some coherent representation of each section. This usually means that there must be some mechanical motion of the apparatus to set up each exposure, for example the distance from the photosensitive hologram plate to the incoherent-coherent means (laser illuminated transparency, liquid crystal screen, and so on) must be varied. Another problem is that in general the hologram results from an incoherent superposition of the separate holograms, since each is recorded independently. This problem is usually solved by making a copy of the original hologram, a process in which one makes a copy of the original poor diffraction efficiency hologram to obtain an efficient final hologram. Thus one requires not only that a separate hologram be made of each section but that the resulting developed hologram must be holographically copied to produce an acceptable final hologram. Synthetic sectional holography has not become commercially popular for many of these reasons.
Sectional holograms are especially useful when the data is naturally represented as a set of points in three dimensions all of which should be visible simultaneously. Examples are sections from biology and medicine, such as results of CAT, MRI and PET scans, or stained sections of anatomical tissue, and so on. Arbitrary 3-D data generated by a computer may also be represented by a section. Recently hologram copying methods, especially embossing methods, have made mass reproduction of holograms economically feasible. It would be useful to be able to generate three dimensional master holograms of educational or commercial value for subsequent distribution, but this usually requires white light viewing. Volume reflection holograms offer one type of white light viewable holograms, and rainbow holograms offer another; rainbow holograms, even as phase relief holograms, may be viewed with appropriate white light illumination so they are especially useful for mass production. Furthermore, rainbow holograms have been shown to offer interesting possibilities for full color holograms even when made with a monochromatic light source. A method and apparatus to produce synthetic rainbow sectional holograms with one exposure would be well suited to producing white light viewable 3-D images and particularly well suited to producing full color synthetic holograms.